Solenoid actuators are well known. In a typical solenoid actuator, an electromagnet includes a wound wire coil surrounding primary and secondary pole pieces. When a current is passed through the windings, an axial magnetic field is generated. A ferromagnetic armature movably disposed in the axial field is urged axially of the windings. The strength of the field, and thus of the actuator, is dependent upon the current at any given voltage. As is well known in the art, the current is inversely proportional to the resistance of the windings. Resistance of the coil can vary with the temperature of the coil. Resistance of the coil can also diminish progressively with length of use.
It is known to use solenoids to actuate fuel injectors for, among others, internal combustion engines and hydrocarbon reformers. Such a fuel injector typically is controlled in known fashion by pulse width modulation (PWM) control of the solenoid actuator; that is, the injector is fully open for a desired fraction of the time of a full injection cycle. Because the resistance and thus the action of a solenoid varies with temperature, it is further known to apply a correction factor to the PWM control, based on coil temperature. For example, if the nominal resistance of a coil is 12 ohms (Ω), but due to ambient temperature and run time the coil resistance is actually 10.5 Ω, then fuel flow through the injector would be greater than desired. At the lesser resistance, current flow is increased and this in turn decreases the opening time and increases the closing time of the fuel injector, effectively increasing total flow in each cycle. Such errors in flow are of special concern for fuel injector operation in open loop during cold engine starts since accurate fuel dispensing is essential to reducing cold-start engine exhaust emissions.
Currently, some known engine management systems (EMS) predict or infer coil temperature from other operating parameters, such as coil duty cycle, voltage, ambient temperature, operation time, and engine temperature, and then alter the pulse bandwidth to compensate for inferred resistance variation from nominal. These approaches share a common shortcoming in relying on inference and not measuring the actual resistance of a solenoid's coil.
What is needed is a means for measuring directly the resistance of a solenoid coil during use thereof, and for adjusting the energizing of the solenoid to compensate for coil resistance deviations from nominal.
It is a principal object of the present invention to provide improved control of a duty cycle of a solenoid.
It is a further object of the invention to improve fuel efficiency of an internal combustion engine.
It is a still further object of the invention to reduce unburned hydrocarbon emissions in engine exhaust.